#include <iostream>
#include <iomanip>
#include <cmath>

class Solution
{
public:
    int main()
    {
        double a, b,c;
        std::cin >> a >> b >> c;
        double p = (a+b+c)/2;
        double area = sqrt(p*(p-a)*(p-b)*(p-c));
        std::cout << std::setprecision(1) << std::fixed << area;
        return 0;
    }
};

#ifndef __LOCAL_TEST__
int main()
{
    return Solution().main();
}
#endif